yeah some big tall back leather chairs are pretty ballin. If I had one of those, I'd definately be kicking back more.
It seems like the whole hunching over thing is pretty much what Armed Baboon said in that it is more of an engrossed in the moment type thing. For me, it seems like I lean more forward when I'm thinking harder or concentrating or getting frustrated at something.
even on my couch I'll sit up and lean forwards but when leaning back, my couch is still a lot more comfy than my studio's chair. I've got a bad lower back (Spondylolisthesis in my L5) that makes finding a nice comfortable office chair tough unless spending big bucks.